Could this be a CalMag deficiency?

The plant looks healthy. You really don't have anything to worry about. I wouldn't recommend adding magnesium or calcium. Just keep an eye on the plant. After transplanting, you can fertilize it since it's growing fast and needs more nitrogen. Remember to maintain the proper pH. Good luck! 👊

what soil did you use? Several come pre fertilized and don't require feeding for 3-4 weeks. If it is prefert soil, don't feed it! You said you transplanted, that's a shock to the plant so could still be recovering. Over watering could cause this but only you know that answer. The color of your leaf veins is more the clue that represents nutrient uptake disruption. I wouldn't feed her, give it some more time to recover Good luck

Add water and Nitrogen, and crank the light to max

I wouldn’t call this a classic Calcium deficiency. The pale color and interveinal chlorosis showing first on the older leaves looks more like Mg/general nutrient hunger, especially since she’s 16 days old and has never been fed. The recent transplant could also temporarily slow uptake, so I wouldn’t react aggressively. Check the pH of your water/root zone, make sure the new 15 L pot isn’t staying constantly wet, and start a light balanced veg feed if the soil itself isn’t strongly pre-fertilized. If the newest growth stays healthy and green, watch it for a few days. If the yellowing keeps moving upward, then Mg/N availability becomes much more likely. CalMag may help if your water is very soft, but I wouldn’t throw a heavy dose at it just because the plant looks pale.

if this is soil and you jsut translplanted, then i wouldn't expect too much of a problem. when you pot up, you got all that new soil, so it's a 'recharge' of nutrients in the pot, so to speak. i doubt it's a soilless grow, but just in case, should be feeding every irrigation in that context. Ca and Mg are two different things and they don't necessarily coincide. They have different symptoms. Download a leaf symptom chart... distinguish between the two. Mg takes 4-6 weaks to show symptoms after the deficiency starts. light might be an inch or 2 too close. If you recently (within a week or so) adjusted this a bit closer or beefed up power, raise it a couple inches... if it's been this height and power for a while, ignore this.

Feed her

I run a similar light, in my opinion the light intensity and distance is aggressive. Secondly, a lot of transplanting and moving these young plants around while they are developing roots. It’s a bit early for any deficiencies

Magnesium this early on is weird is it not?

I know the grow world combines them into one thing and makes it confusing to new people but you need to think of them as seperate things. CAlmag is a product. Plants use calcium and magnesium if that makes sense. Epsom salt is the shit!

Thats magnesium, Epsom salt to the rescue !

I have the same on leaves of one plant in a tent of 3. Arrived at the same conclusion you have so added a little cal mag in the water each feed. I have started LST so I removed the offending leaves. Will see how the new leaves come out. Good luck. P.S I usually add some Dolomite Lime mixed well in my substrate that usually helps but I forgot this time round.